[PATCH v2 5/8] p5303: measure time to repack with keep

From: Jeff King <redacted>

This is the same as the regular repack test, except that we mark the
single base pack as "kept" and use --assume-kept-packs-closed. The
theory is that this should be faster than the normal repack, because
we'll have fewer objects to traverse and process.

Here are some timings on a recent clone of the kernel. In the
single-pack case, there is nothing do since there are no non-excluded
packs:

  5303.5: repack (1)                          57.42(54.88+10.64)
  5303.6: repack with --stdin-packs (1)       0.01(0.01+0.00)

and in the 50-pack case, it is much faster to use `--stdin-packs`, since
we avoid having to consider any objects in the excluded pack:

  5303.10: repack (50)                        71.26(88.24+4.96)
  5303.11: repack with --stdin-packs (50)     3.49(11.82+0.28)

but our improvements vanish as we approach 1000 packs.

  5303.15: repack (1000)                      215.64(491.33+14.80)
  5303.16: repack with --stdin-packs (1000)   198.79(380.51+7.97)

That's because the code paths around handling .keep files are known to
scale badly; they look in every single pack file to find each object.
Our solution to that was to notice that most repos don't have keep
files, and to make that case a fast path. But as soon as you add a
single .keep, that part of pack-objects slows down again (even if we
have fewer objects total to look at).

Signed-off-by: Jeff King <redacted>
Signed-off-by: Taylor Blau <redacted>
---
 t/perf/p5303-many-packs.sh | 22 ++++++++++++++++++++--
 1 file changed, 20 insertions(+), 2 deletions(-)

diff --git a/t/perf/p5303-many-packs.sh b/t/perf/p5303-many-packs.sh
index d90d714923..b76a6efe00 100755
--- a/t/perf/p5303-many-packs.sh
+++ b/t/perf/p5303-many-packs.sh

@@ -31,8 +31,11 @@ repack_into_n () {
 	' "$1" >pushes &&
 
 	# create base packfile
-	head -n 1 pushes |
-	git pack-objects --delta-base-offset --revs staging/pack &&
+	base_pack=$(
+		head -n 1 pushes |
+		git pack-objects --delta-base-offset --revs staging/pack
+	) &&
+	test_export base_pack &&
 
 	# and then incrementals between each pair of commits
 	last= &&

@@ -49,6 +52,12 @@ repack_into_n () {
 		last=$rev
 	done <pushes &&
 
+	(
+		find staging -type f -name 'pack-*.pack' |
+			xargs -n 1 basename | grep -v "$base_pack" &&
+		printf "^pack-%s.pack\n" $base_pack
+	) >stdin.packs
+
 	# and install the whole thing
 	rm -f .git/objects/pack/* &&
 	mv staging/* .git/objects/pack/

@@ -91,6 +100,15 @@ do
 		  --reflog --indexed-objects --delta-base-offset \
 		  --stdout </dev/null >/dev/null
 	'
+
+	test_perf "repack with --stdin-packs ($nr_packs)" '
+		git pack-objects \
+		  --keep-true-parents \
+		  --stdin-packs \
+		  --non-empty \
+		  --delta-base-offset \
+		  --stdout <stdin.packs >/dev/null
+	'
 done
 
 # Measure pack loading with 10,000 packs.

-- 
2.30.0.533.g2f8b6b552f.dirty